1. Field of the Invention
The present invention relates to a method for manufacturing a wiring substrate of a multilayer structure including vias.
2. Description of the Related Art
In recent years, concomitant with the trend toward high density mounting of components, a multilayer wiring substrate in which a plurality of wiring patterns is formed to have a multilayer structure has been used. As a method for manufacturing the multilayer wiring substrate as described above, Japanese Unexamined Patent Application Publication No. 9-36551 disclosed a technique in which a wiring pattern is formed on one surface of a hard substrate, an adhesive layer is formed on the other surface thereof, holes penetrating the hard substrate and the adhesive layer are formed to be in contact with the wiring pattern, and a conductive paste is filled in the above holes.
FIG. 11A shows one example of the manufacturing method disclosed in Japanese Unexamined Patent Application Publication No. 9-36551. As shown in FIG. 11A-(a), a hard resin substrate 100 provided with a metal foil 101 adhered on an upper surface thereof is prepared, and the metal foil 101 is processed by an etching treatment to form a wiring pattern 101a as shown in FIG. 11A-(b). Next, as shown in FIG. 11A-(c), an adhesive layer 102 is formed on one surface of the resin substrate 100, and subsequently by radiating laser from an adhesive layer side as shown in FIG. 11A-(d), continuous via holes 103 each penetrating the adhesive layer 102 and the resin substrate 100 are formed. In addition, as shown in FIG. 11A-(e), a conductive paste 104 is filled in the via holes 103, so that a single-sided circuit board can be obtained. At this stage, the adhesive layer 102 and the conductive paste 104 are both in an uncured state.
After a plurality of single-sided circuit substrates 105a to 105d which are formed by a method similar to that described above are laminated to each other as shown in FIG. 11B, the adhesive layers 102 and the conductive pastes 104 are simultaneously heat-cured, so that a multilayer wiring substrate as shown in FIG. 12 can be obtained.
When the via holes 103 each having a bottom made of the wiring pattern 101a are formed in the resin substrate 100 by laser processing as described above, a problem may arise in that the via holes 103 each have a tapered shape. The reason for this is that, in the via hole 103 having a bottom, since laser light must be weak to prevent the wiring pattern 101a which forms the bottom of the via hole 103 from being damaged by the laser light, energy of laser light reaching the bottom of the via hole 103 is reduced to a very low level. In the case of the via hole 103 having a tapered shape, since the diameter of the bottom of the via hole is small, in order to prevent a connection defect between the conductive paste 104 and the wiring pattern 101a at the bottom of the via hole, it is necessary to increase the diameter of an opening portion of the via hole 103. As a result, the pitch between the vias cannot be narrowed, and hence, formation of fine wiring is disadvantageously hindered. In particular, when the resin substrate 100 is a component-incorporating substrate, that is, when the resin substrate 100 incorporates a circuit component therein, since the thickness of the substrate is increased, the diameter of the opening portion of the via hole 103 is further increased.
In addition, in the conventional manufacturing method described above, since a laser is radiated to the resin substrate 100 provided with the adhesive layer 102 adhered thereto as shown in FIG. 11A-(d), the adhesive layer 102 in an uncured state is melted by heat, and hence the diameter of the via hole 103 in the adhesive layer 102 is unnecessarily increased. The increase in diameter as described above further hinders the formation of fine wiring in combination with the tapered shape of the via hole 103.
Furthermore, since a resin removed by laser radiation may adhere to the peripheries of the via holes 103 and/or the surface of the wiring pattern 101a, for example, a desmear treatment or a plasma treatment is necessarily performed to remove the above-described resin. However, by the method for removing smears described above, the uncured adhesive layer 102 is also simultaneously removed. As a result, smears are not appropriately removed, and hence electrical reliability may be degraded in some cases.